1. Field of the Invention
The present invention generally relates to a protection apparatus for a brushless motor, and more specifically, to such a protection apparatus used for a brushless motor in the case that the brushless motor is employed as a fan motor, for instance, a fan of an outdoor unit of an air conditioner is driven by this brushless motor.
2. Description of the Related Art
Conventionally, induction motors are widely utilized as motors for driving outdoor fans. However, very recently, brushless motors operable in high efficiencies have been gradually introduced instead of these induction motors, while a strong demand is made of air conditioners operable in high efficiencies. FIG. 19 represents one of drive apparatus such that a brushless motor is driven as a fan motor.
This drive apparatus includes a brushless motor 1001, a fan 1002, a drive circuit 1003, an output voltage instructing circuit 1004, a commutation sensor 1005, a commercial power supply 1006, a rectifying circuit 1007, and a smoothing capacitor 1008.
The commercial power supply 1006 outputs an AC voltage of 100V, or 200V. The AC voltage outputted from the commercial power supply 1006 is rectified/smoothed by the rectifying circuit 1007 and the smoothing capacitor 1008 so as to be converted into a DC voltage of 140V, or 280V. Thereafter, this converted DC voltage is entered as an output voltage of a DC main power supply into the drive circuit 1003. The fan 1002 is coupled to an output shaft of the brushless motor 1001. This brushless motor 1001 is provided with the commutation sensor 1005 for sensing a position of a movable member, or a rotor (not shown). The drive circuit 1003 produces a commutation signal used to drive the movable member of the brushless motor 1001 based upon the output signal from the commutation sensor 1005, and thus drives the brushless motor 1001. Also, the drive circuit 1003 outputs the output voltage of the DC main power supply in a ratio defined in response to the output voltage instruction signal derived from the output voltage instructing circuit 1004. In other words, the drive circuit 1003 is operated in a so-called "PWM (pulse width modulation) system" such that in response to the output voltage instruction signal, the ratio of the time during which the respective drive windings of the brushless motor 1001 are connected to the plus-sided voltage terminal (DC 140V, or DC 280V) of the DC main power supply to the time during which the respective drive windings thereof are connected to the minus-sided voltage terminal (DC 0V) of the DC main power supply is controlled so as to vary the drive voltage of the brushless motor 1001.
However, in such a case that this drive apparatus is applied to the outdoor fan of the air conditioner, when this outdoor fan is forcibly and externally driven by receiving a strong wind force such as typhoon or cyclone, the below-mentioned difficulties would occur.
That is, when such a wind (adverse, or against wind) is produced which may drive the fan 1002 along the direction opposite to the drive direction of the brushless motor 1001, this brushless motor 1001 is brought into the overload state, so that the drive current thereof is increased. Also, when the brushless motor 1001 is rotated along the reverse direction, the drive current thereof may be further increased due to the influences caused by the induced voltage from the brushless motor. When the drive current of this brushless motor 1001 is increased, the amounts of heat generations of this brushless motor 1001 and the drive circuit 1003 for supplying the power to this brushless motor 1001 are increased. In the worst case, both the brushless motor 1001 and the drive circuit 1003 would be electrically destroyed. To avoid the above-explained heat generations and electrical destroy of the brushless motor 1001 and the drive circuit 1003, the following tripping method has been proposed in which either the overheat or the overcurrent of the brushless motor 1001 and the drive circuit 1003 is detected to interrupt the supply of power. Therefore, these circuit elements are not self-recovered. However, for example, such a fan motor which is frequently tripped to be stopped every time typhoon approaches could not be employed as an outdoor fan motor of an air conditioner.
On the other hand, in the case that such a wind (fair, or following wind) is produced which may drive the fan 1002 along the same direction as the drive direction of the brushless motor 1001, the brushless motor 1001 is forcibly accelerated, and thus while the rotation speed is increased, the induced voltage internally produced in the brushless motor 1001 is also increased, so that the brushless motor 1001 will produce the regenerative voltage. The generation of this regenerative voltage is determined based on such a relationship between the induced voltage internally produced from the brushless motor 1001 and the output voltage outputted from the drive circuit 1003 as the drive voltage for this brushless motor 1001. If this induced voltage becomes higher than this output voltage, then the regenerative voltage is produced. As a consequence, the regenerative voltage is produced when the above-described induced voltage becomes higher than the above-mentioned output voltage irrelevant to the magnitudes of the voltage values of the output voltages derived from the drive circuit 1003, which is determined based on the output voltage instruction signal supplied from the output voltage instructing circuit 1004. When the regenerative voltage of the brushless motor 1001 is produced, the regenerative power thereof is supplied to the drive circuit 1003. As a result, this regenerative power could increase the voltage across the terminals of the smoothing capacitor 1008. When the voltage across the terminals of the smoothing capacitor 1008 is increased, both the smoothing capacitor 1008 and the drive circuit 1003 are brought into the overvoltage states. Accordingly, there is a risk that these smoothing capacitor and drive circuit are electrically destroyed.